[Cashtab] Better testing of useWallet hook

Summary:
Add more tests for the useWallet hook.

This diff introduces automated testing of localforage interaction. This will provide a baseline for future refactors of useWallet.js, demonstrating that expected behavior is preserved.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15351